[voischev]

Summary

Add a standalone AGENTS.md for renderer development and coordinated parser/core work.

• Describe the existing AVA/XO/clinton/tsup toolchain and Node.js 12/14/16 CI baseline using the actual manifest.
• Record exact-output contracts for quoting, JSON/boolean attributes, raw text, void tags and tag: false unwrapping.
• Explain the parser's development/type-only relationship and keep runtime requirements tied to actual runtime dependencies.
• Document tarball-based integration checks and release ordering, with the canonical protocol in Integrate parser 0.13 and cross-package agent guides posthtml#410.
• Exempt only AGENTS.md from Clinton's file checks so the conventional uppercase instruction filename is accepted.

Verification

Instructions were checked against the existing manifest, CI, source and test layout. git diff --check passes.

The legacy CI has a pre-existing Clinton pkg-schema failure resolving external SchemaStore references. This was reproduced with Clinton 0.14.0 on Node.js 12 in an isolated worktree of unmodified master 47e750b: the same ten schema-resolution errors occur before XO/AVA can run. With this PR's filename exception, the feature branch reports the same baseline errors and no AGENTS.md filename error. The schema rule is preserved.

This PR changes contributor instructions and their legacy filename-validation exception; it requires no npm release. Runtime source, dependencies and the supported Node.js minimum are unchanged. Toolchain modernization remains a separately scoped change.